In passenger tires and heavy-load tires used for trucks, buses, etc., it has been required from the safety and economy viewpoints to improve abrasion resistance, cut resistance, and chipping resistance, and improvements of rubber compositions used for tires have been made. Sulfur, as a vulcanizing agent, is commonly used for such rubber compositions for tires. However, it is known that in vulcanized rubbers obtained by vulcanizing natural rubber, etc. with sulfur, crosslinked sulfur chains are cleaved and re-crosslinked by thermal fatigue, and thus the crosslink length is shortened.
For this reason, problematically, heat aging causes the rubbers to be harder, and causes the tensile strength, abrasion resistance, etc. to decrease, resulting in deterioration of tire performance. Also problematically, in sulfur crosslinking, the phenomenon of reversion easily occurs and leads to inferior stability of rubber physical properties.
As a rubber composition for a tire tread that inhibits the deterioration of rubber physical properties caused by reversion or heat aging, Patent Document 1 discloses a rubber composition containing a rubber component, such as natural rubber, in combination with sulfur and an anti-reversion agent, such as 1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane.
However, the combined use of sulfur and such an anti-reversion agent extends scorch time, results in longer vulcanization time, and thereby may cause a decrease in productivity. Additionally, since the rubber composition contains sulfur crosslinking, the cleavage and re-crosslinking of sulfur chains caused by thermal fatigue are unavoidable. Therefore, the rubber physical properties are more likely to change, and the performances such as abrasion resistance are not sufficient. Moreover, there has recently been a growing demand for fuel economy of tires.    Patent Document 1: JP-A 2006-45471